A crank angle detector for an engine includes a cam rotor plate for detecting a cylinder number to be ignited and a cam pulse sensor provided opposite thereto, a crank rotor plate for sensing a crank angle and a crank pulse sensor provided opposite thereto, and a controller for determining ignition timings of respective cylinders to control an ignition. The crank rotor plate is constituted by a rotor plate at starting for sensing a fixed ignition timing and a rotor plate for a normal operation. A pair of crank pulse sensors are provided opposite to the rotor plates, respectively. An input signal for the fixed ignition timing is mask-released only at starting. After that, input is continuously masked during normal operation.